NetworkProvider: add missing synchronization am: aecfab5f72 am: 617e2ba73a

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/2148673

Change-Id: Id9971c324702e520fade03c52a9cf8e422b7a658
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>